86 C. A. L. Bassett 



though under certain circumstances they may be predominantly uniphasic. In the 

 light of this observation, it seems reasonable to ask whether a cell can discriminate 

 between the positive and negative phases of the signal and act accordingly? Or, does 

 it react to the greater or lesser electrical activity, produced by greater or lesser stress? 

 Although concrete answers are not available presently, a working hypothesis has 

 been devised. Such an hypothesis should explain how, despite the stimulus of systemic 

 hormonal and nutritional factors, mesenchymal cells can specialize simultaneously as 

 osteoblasts and osteoclasts within a few micra of one another. 



Generally speaking, when a potential difference is measured between two surfaces, 

 the relative availability of freely mobile electrons is being detected. Under these 

 circumstances, positivity may be comparable to cold, i.e., cold represents an absence 

 of heat, while positivity represents a lack of electrons. If this definition is permissible, 

 then osteoclasis may result when electrical activity is diminished or non-existant. 

 This line of reasoning can lead in any of several directions. For example, diffusion of 

 extracellular fluids in bone probably is less efficient than in any other connective 

 tissue. Bone substance is relatively incompressible, so that fluids cannot be "massaged" 

 back and forth. Canaliculae, through which nutrient fluids must diffuse, comprise, at 

 most, only 30/0 of the total area of a lacunar space. Furthermore, many osteocytes 

 are situated at relatively great distances from their blood supply. In the face of such 

 an inefficient supply line, it might be expected that most osteocytes would be on the 

 brink of starvation or, at least, of anaerobic glycolysis. Under the impetus of minor, 

 physiologic skeletal deformations, however, an alternating signal could aid electro- 

 phoretically in the ebb and flow of charged molecules and ions, thereby aiding 

 greatly in the nutrition of these cells (Fig. 4) (Bassett, in press). If the locus for 

 electrogenesis proves to be collagen-apatite junction, then the average osteocyte is 

 surrounded by more than 1.1 XIO'* generating loci! 



If electrogenesis were increased above maintenance levels in a given region, local 

 cells might be activated to produce bone to stabilize the regions. Conversely, 

 diminished electrical activity might be followed by osteocyte death through starvation. 

 It should be re-emphasized here that the stress generated potentials apparently are 

 not dependent upon cell viability and, therefore, dead bone may continue to generate 

 potentials if subjected to intermittant deformation and if it remains electrically com- 

 petent. Should electrical activity cease in a region, osteoclasis would result. This 

 hypothesis seems to fit well with the observed behavior of teeth in orthodontia. 

 Normally, a tooth is suspended in its socket by a periodontal membrane comprised of 

 collagen bundles that penetrate the surrounding bone, much as Sharpey's fibers would 

 (Fig. 5). When orthodontists move a tooth with 1 — 2 grams of steady, lateral pressure, 

 the periodontal fibers become taut on the trailing edge of the root, where osteoblasts 

 appear to deposit layers of appositional bone. On the leading edge, where the fibers 

 are slack, osteoclasts are active, removing bone. Similar osteoblastic behavior has been 

 observed during regeneration of bone in Millipore-isolated defects in dog radii 

 (Bassett et al., 1961). Unless stimulated by major instability, these defects fill rapidly 

 with fibrous tissue which gradually is converted directly to bone in a centripetal 

 manner. The interface between bone and fibrous tissue is spanned by many collagen 

 bundles that are oriented along major lines of stress. At the point where they emerge 

 from the osseous tissue, rows of active osteoblasts are found (Ruedi and Bassett, 

 unpublished). This picture is not greatly different from that in ossifying turkey 



